A fuel cell such as a polymer electrolyte fuel cell is expected to expand in the future as a power source for a vehicle or a stationary generator. However, according to a known polymer electrolyte fuel cell, a defect such as a small hole formed in a polymer electrolyte membrane used as an electrolyte may be caused when the fuel cell is continuously operated. For example, it is widely known that hydrogen used as a fuel and oxygen used as an oxidant leak to respective counter electrode sides by passing through the polymer electrolyte membrane, and that with the passage of time the amount of leaking increases. When the amount of leaking exceeds a predetermined level, a danger arises of not only a decrease in cell output but also of combustion and explosions of gas that has passed through the polymer electrolyte membrane, thus preventing the fuel cell from operating. In these circumstances, a joint member of the polymer electrolyte membrane and the electrode needs to be replaced by a new one, or an entire fuel cell needs to be replaced.
In addition to the leaking of gas, a reduction in the quality of a polymer electrolyte membrane can occur as a result of an oxidative degradation or the like concomitant with the passage of operation time of the fuel cell. Proton conductivity is thereby decreased, resulting in a decrease in cell output. In order to extend a fuel cell life, it is necessary to clarify the reasons why a defect, or a change in quality may occur in the polymer electrolyte membrane.
It is highly desirable that hydrogen and oxygen supplied to the fuel cell do not permeate the polymer electrolyte membrane. However, in practice, considerable permeation may occur.
A greater part of oxygen permeating a counter electrode, i.e. the fuel electrode, from the oxidant electrode becomes water by reacting with hydrogen in the fuel electrode. At this time, however, since an electrode potential of the fuel electrode is low, hydrogen peroxide is generated, a process which causes damage to the polymer electrolyte membrane. Moreover, in the even of hydrogen permeating a counter electrode, i.e. the oxidant electrode, from the fuel electrode, a reaction speed of hydrogen with oxygen for generating water decreases since in a case where an electrode potential of the oxidant electrode is high, a catalytic property of a platinum catalyst used for the oxidant electrode relative to hydrogen oxidation decreases. Therefore, a state occurs in which hydrogen and oxygen exist in the oxidant electrode. In this state, if the oxidant electrode potential is lowered, hydrogen and oxygen rapidly react with each other, thereby causing damage to the polymer electrolyte membrane.
Thus, a need exists for a fuel cell in which a defect, or change in quality, in a solid electrolyte can be prevented.
Further, in order to prevent defects or changes in quality in a polymer electrolyte membrane, both hydrogen and oxygen should be prevented from passing through the polymer electrolyte membrane and thus reaching the respective counter electrodes. The present invention has been made in view of the above circumstances and provides such a fuel cell.